1. Field of the Invention
The present invention relates to a data slicer and its operating method, and more particularly, to a data slicer capable of automatically removing the current mismatch between current pumps incorporated therein and its operating method.
2. Description of the Prior Art
With a rapid development of computer technology, most analog data can be transformed into digital data to facilitate data transmission and storage. In recent years, the use of compact discs (CDs) as a storage medium has been adopted extensively. As a result, optical recorders such as recordable compact disc (CD-R) and rewritable compact disc (CD-RW) drives have entered the mainstream of the electronic product market. A great amount of information can be stored on a compact disc through the use of these optical recorders.
Please refer to FIG. 1 which shows a top view of a typical compact disc 10. As is well known in the art, the compact disc 10 is provided with a reflecting surface 13. Generally a compact disc drive uses an optical pick-up head to emit a laser beam onto the reflecting surface 13 of the compact disc 10, and the incident laser beam is further reflected by different parts of the reflecting surface 13. The compact disc drive reads the information retained in the compact disc 10 by using the optical pick-up head to collect the reflected laser beam. That is, the compact disc drive can transform optical signals into corresponding electronic signals. On the reflecting surface 13 of the compact disc 10 is a fine spiral track 11. Taking into account a recordable compact disc (CD-R) and a rewritable compact disc (CD-RW), please refer to FIG. 2 which is a magnified view of the area 1A taken from FIG. 1 if a recordable compact disc (CD-R) or a rewritable compact disc (CD-RW) is selected as the compact disc 10 of FIG. 1. In FIG. 2, the track 11 is composed of two types of tracks, one being a data track 12 adapted to record data and the other being a wobble track 14 adpated to record related time information of each data frame. The data track 12 has a continuously spiral shape, and the wobble track 14 has an oscillating shape as shown in FIG. 2. Additionally, the curvature of the wobble track 14 is composed of small segment curves with different periods. The wobble track 14 is used to generate a corresponding wobble signal. Because the wobble track 14 is composed of small segment curves with two different periods, the wobble signal is composed of signal segments with two different frequencies. It is well known that the absolute time in pre-groove (ATIP) information is modulated by frequency modulation (FM). Therefore, the wobble signal can be demodulated to recover the ATIP information that is used to record information such as minutes, seconds, and data frames related to each data track 12. The surface of the wobble track 14 protrudes from the reflecting surface 13, and the data track 12 is located inside a groove formed by the protruding wobble track 14 as shown in FIG. 2. The data track 12 has a plurality of pit areas 16 and a plurality of land areas 18. Each pit area 16 and land area 18 are used to represent digital data “1” and “0” respectively.
Please refer to FIG. 3, which is a diagram showing the detection of reflected laser signal from the data track 12 as shown in FIG. 1. When the optical pick-up head emits a laser beam with a predetermined radiation power on the data track 12, the optical pick-up head simultaneously detects a reflected laser beam. If the optical pick-up head moves to the pit areas 16, the emitted laser beam is scattered. Therefore, the radiation power of the reflected laser beam detected by the optical pick-up head is weaker. On the contrary, if the pick-up head moves to the land areas 18, the emitted laser beam is mostly reflected. Therefore, the radiation power of the reflected laser beam here about detected by the optical pick-up head is greater than that generated from the pit areas 16. The optical pick-up head generates a detecting signal 20 according to the radiation power of the reflected laser beam. It is well known that the detecting signal 20 is an AC coupled RF signal. As shown in FIG. 3, each of the pit area 16 corresponds to a portion of the detecting signal 20 having a negative amplitude as compared with a DC level, and each of the land area 18 corresponds to a portion of the detecting signal 20 having a positive amplitude as compared with the DC level. The DC level is a long-term average of amplitudes of the detecting signal 20. The digital data stored on the data track 12 have been encoded according to a predetermined method so that a corresponding digital sum value (DSV) approaches 0. In other words, the total number of “1” and the total number of. “0” ideally should be equal to make the digital sum value approach 0. The total length of pit areas 16 on the data track 12, therefore, will be equal to the total length of land areas 18 on the data track 12 so as to make the DSV equal to 0.
When the optical pick-up head reads data from the compact disc 10 of FIG. 1, the optical pick-up head accordingly generates the analog detecting signal 20. A data slicer is then widely used to convert the analog detecting signal 20 into corresponding digital data. Please refer to FIG. 4, which is a circuit diagram of a prior art data slicer 30. The data slicer 30 has a comparator 32, two current pumps 34, 36, an inverter 35, a capacitor 37, a low pass filter (LPF) 38, and two switches 41, 42. The LPF 38 includes a resistor 39 and a capacitor 40. The comparator 32 has one input terminal (non-inverting input terminal) electrically connected to the detecting signal 20, and another input terminal (inverting input terminal) electrically connected to the output of LPF 38 for receiving a slice reference level Vr. The comparator 32 is used to compare the detecting signal 20 with the slice reference level Vr. If the amplitude of the detecting signal 20 is greater than the slice reference level Vr, the comparator 32 outputs a high voltage level representing a logic high value (“1” for example). The switch 41 is turned on accordingly, but switch 42 is turned off owing to the inverter 35. The current pump 34 starts charging the capacitor 37 so as to increase the slice reference level Vr by a first offset value. If the amplitude of the detecting signal 20 is less than the slice reference level Vr, the comparator 32 outputs a low voltage level representing a logic low value (“0” for example). The switch 42 is turned on owing to the inverter 35, but the switch 41 is turned off. The current pump 36 starts discharging the capacitor 37 so as to decrease the slice reference level Vr by a second offset value. The LPF 38 functions as an integrator to adjust the slice reference level Vr according to the operations of current pumps 34, 36. In the prior art data slicer 30, the current pumps 34, 36 are supposed to be identical. That is, the first offset value should be equal to the second offset value. However, current pump 34 is not identical to current pump 36 even both are fabricated by the same semiconductor process. There is a mismatch between the current pumps 34, 36. When the current pumps 34, 36 are turned on by the same control voltage, the first offset value is possibly not identical to the second offset value. Is this way, the slice reference level Vr will be shifted upward or downward after a long period of time.
Please refer to FIG. 5, which is a diagram showing the mismatch between the current pumps 34, 36. The detecting signal 20, which is an analog RF signal, is inputted into the comparator 32 of the data slicer 30. Suppose that the digital data retained in the compact disc 10 corresponds to a “zero-DSV”. When the corresponding detecting signal 20 is sliced by the data slicer 30 to reproduce the original digital data, the DSV relating to the reproduced digital data should be 0. If the current pumps 34, 36 are identical and have the same circuit characteristic, the long-term average of the slice reference level Vr approaches LV1 shown in FIG. 5. It is obvious that the reproduced digital data are “11111111110000000000”. The total number of “1”s is equal to the total number of “0”s. The DSV, therefore, is equal to 0. If the first off set value is greater than the second offset value, the charging effect on the capacitor 37 is more powerful than the discharging effect on the capacitor 37 after a long period of time. The long-term average of the slice reference level Vr is then shifted upward from LV1 (ideal value) to LV2. It is obvious that the reproduced digital data are “01111111100000000000”. The total number of “1”s is less than the total number of “0”s. The DSV of the digital data, therefore, is equal to a negative number (−2 for example). If the first offset value is less than the second offset value, the discharging effect on the capacitor 37 is more powerful than the charging effect on the capacitor 37 after a long period of time. The long-term average of the slice reference level Vr is then shifted downward from LV1 (ideal value) to LV3. It is obvious that the reproduced digital data are “1111111111000000001”. The total number of “1”s is greater than the total number of “0”s. The DSV of the digital data, therefore, is equal to a positive number (2 for example).
Because there is a mismatch between the current pumps 34, 36, the actual reference level Vr is deviated from an ideal value so that the DSV of the reproduced digital data runs out of a reasonable tolerance window. The total number of the error bits in the reproduced digital data increases the loading of the following error correction circuit designed to recover the original digital data retained on the compact disc 10. In other words, the mismatch between the current pumps 34, 36 greatly affects the accuracy of the output data generated from the data slicer 30. The performance of the data slicer 30 is then deteriorated.